Leaellynasaura (meaning "Leaellyn's lizard") was a fast-running, plant-eating dinosaur that lived in Australia during the middle Cretaceous Period, roughly 115-110 million years ago. Australia was within the Antarctic Circle during the Cretaceous period, and it had a very long winter.

Leaellynasaura is known from two nearly complete skeletons and two skulls. It is described as an ornithopod.[1] Unlike some other ornithischians, the Leaellynasaura did not have any tendons in its tail. The tail is known for being among the longest of its relatives: it was three times longer than the rest of the whole body. It also has more vertebrae in its tail than any other ornithischians except for some hadrosaurs.[2] It also had large eyes to help it look in the dark winter.